a black spot is situated at the body center of a transparent glass

( n=1.5 ) cube of edge 0.1m,placed on a table.what minimum fraction of the cube must be covered in order that the spot becomes invisible ?

Have a look into the image, which shows the area that needs to cover


from this we get the radius of the area to be covered.
tan(i)=2.25/3
the radius=3.75 cm

the area required is 6*area of this circle.=264.9375 cm²